The invention is directed to a method and apparatus for coating selected portions of a printed circuit board with a solder resist material in order to mask those portions from subsequently applied solder. After wave soldering or the like, an aqueous cleaner is used to clean the soldering resin and the solder masking material from the circuit board.
It is known to contact selected portions of a circuit board with the needle of a pressurized syringe or pot in order to apply a solder resist material thereto. The above cross-referenced U.S. Pat. No. 4,059,708, discloses the use of a hypodermic syringe in applying masking drops to a substrate, although not disclosing solder masking.
U.S. Pat. No. 3,053,215 discloses the use of liquid wax as a solder resist material which is sprayed onto a circuit board as it is being conveyed to a solder bath.
U.S. Pat. No. 4,230,793 discloses applying an ultra-violet light curable liquid photopolymer in a free falling curtain of material onto the top of a circuit board being conveyed, irradiating and curing portions of the photopolymer which are not to be soldered, and the subsquent developing of the cured layer with an agent which dissolves the unirradiated areas.
U.S. Pat. No. 4,373,655 discloses the concept of inserting mechanical devices into holes of a circuit board prior to subjecting the circuit board to wave soldering or the like.
U.S. Pat. Nos. 4,246,147 and 4,301,189 relate to masking selected portions of a circuit board by a screen printing process.
It is an object of the instant invention to provide a method and apparatus for selectively coating portions of a circuit board with a solder resist material wherein the resist material is projected through the air from a gun tip to selected portions of the circuit board, thus obviating the need for contacting the circuit board with the applicator.
It is a further object of this invention to project shots of a masking material onto selected portions of the circuit board wherein relative positioning between the projector and the circuit board and/or the amount of material projected onto the circuit board is programmably controllable and variable, thus obviating the need for a screen as in screen printing and for the changing of screens in order to vary masking of selected portions of the circuit board.
Still further, it is an object of this invention to alleviate the prior art problems of air entrapment in the solder masking material. By applying a relatively high pressure to the material, any air entrapped in the material is dispersed throughout the material for an homogeneity thereof. In practicing the invention, large air bubbles do not form in the tip of the dispensing gun and there is a consistent density ratio of air to solder masking material.
It is an additional object of this invention to obviate the sometimes lengthy process of running a vacuum on the masking material in an attempt to degas or remove entrapped air therefrom, since such degasing or deaerating is only 80% efficient.
It is another object of the instant invention to detect proper projection of solder masking material from a high speed valving gun, and for cleaning the tip of the gun upon clogging thereof.
Additionally, it is an object of the invention to ensure proper projection of shots of the solder masking material onto the circuit board by shielding ambient air currents from the general area of the projector at which consecutive shots of material are separated.
In accordance with one embodiment of the invention, the solder masking material is pressurized by means of a fluid pump and a regulator to approximately 250 psi, and a high speed valve is used to turn the flow on and off rapidly in order that small "shots" of fluid are propelled from a tip, through the air, to selected portions of a circuit board. The selected areas of the circuit board to be masked are situated oppositely from the tip, as by an X-Y positioning system for the tip and/or the circuit board. The drops of masking material, which form upon impinging of the shots onto the circuit board, may be varied in volume and diameter. Where needed, a continous bead of masking material may be formed on the circuit board by spacing the dots sufficiently close together.